Federal/Military Europe

I'm not sure I see the point, unless you're thinking expeditionary.

There are only five countries in the EU which are not in NATO: Sweden, Ireland, Finland, Cyprus and Malta, so the chances are that anything which will be going to trigger the EU's mutual defense pact is also going to trigger NATO.

https://euobserver.com/news/119894
Witney, who used to run the EU's military research branch, the European Defence Agency in Brussels, mentioned three reasons for lack of EU-level progress.

"Firstly, lack of money. Secondly, the fact everybody feels safe … and thirdly because most people in Europe don't take defence terribly seriously. The Poles do a bit, and the Brits and the French. But mostly, people are interested in what their defence budget can do for their industrial base or for regional unemployment," he said.

Another reason is Nato itself. During the Russian simulation in March, Sweden's air force did not react because it was on low alert during the Easter break. But Nato scrambled two Danish jets from a base in Lithuania.

"Nato is always there and it has the Americans on board, which is quite handy," Witney added. He noted that even if Nato's article five does not cover non-Nato-members and even if Lisbon's 42.7 is fanciful, there is an overriding "solidarity of fact," however.
He said: "Nato countries and EU countries are so dependent on one another, their interests run into each other so much … no one is going to attack Stockholm and think that British forces or other Nato forces will stand aside and watch."

With ref to Johnnyskeleton's commentary about Trump, for all his noise about threatening to leave NATO if the member states don't pony up more cash, the reality is that the US, even under Trump, is still well committed to NATO. The US Army just last month had its largest exercise in Europe since the end of the Cold War, snap-activating five times as many transport ships as normal, and involving a large number of reserve units, comparison to the 1980s era REFORGER exercises are common. Next year's will be even bigger. https://breakingdefense.com/2019/10/massive-nato-wargame-acquisitions-seek-to-shore-up-fraying-alliance/

Next spring, the Army will deploy 20,000 US-based soldiers to Europe to operate alongside 17,000 allied troops in the massive Defender-Europe 20 exercise. It will be the US Army’s largest troop movement in 25 years, spreading its forces across Central Europe, Eastern Europe, and Georgia in a test, not only of Army logistics, but the ability of US forces to link up with allies.

Defender 20 is just the latest in a series of massive, multinational exercises in Europe over the past year that emphasize how — despite international anxiety over President Trump’s mercurial policy shifts — the Pentagon is doubling down on its traditional alliances.

So the bottom line is that as long as you're NATO-compatible, you'll be able to work reasonably well. And if your focus is just on EU,you still need to be NATO compatible so that you can work with the other EU militaries which are NATO compatible. Witness, for example, the Swedish military redefining a circle in 2007 to match with the NATO standard. (From 6,300 mils to 6,400 mils) so that they can work as part of the Nordic battlegroup. Witness also how the French pulled, militarily, out of NATO in 1966 (Coming back only in 2008 or so), but given the unspoken assumption that if the Soviets invaded they would fall back in, they trained and equipped to fight as part of NATO anyway.

So, we can see that, in terms of a Europe-wide military defense, NATO is basically the default. Little infringements like a Russian bomber crossing over Donegal is something that a country needs to sort out for itself, or if you're Ireland, make an agreement with another country to do it. That just leaves expeditionary operations. What sort of expedition would the EU, as a unified group, undertake, as opposed to a coalition of forces such as we currently use? Bearing in mind that the EU is not a unified body with a common foreign policy. True unity of command such as a single force like NATO has is really only necessary in a high-intensity operation... the sort of expeditionary operation which Ireland's triple-lock would probably prohibit. In which case, would Europe's Third Infantry Division (Western Europe) really be able to operate and train effectively if they can't even guarantee that one of its brigades (or a divisional supporting unit or some such) is even going to show up for the fight?

When it comes down to multi-national development of major end items like ships, tanks, airplanes, etc, decades of history has shown that those that can afford to develop independently do so. Not so much for job protection: It is entirely possible for your domestic factory to build another nation's design: Witness Egypt building M1 Abrams, or Turkey, Belgium and the Netherlands building F-16 jets, or even the US tank using a German cannon. The British can certainly design their own helicopters, but British Army Apaches are built in Somerset. The difficulty is that each nation has their own requirements. This article explains the problems of the German/US MBT-70 tank project, for example. https://apps.dtic.mil/dtic/tr/fulltext/u2/a105123.pdf

Ireland can't afford expensive aircraft, it might lean to single-engined jets like F-16 or Gripen. Australia has stupidly large areas to cover on both land and water, they insisted that the redundancy provided by twin engines was mandatory and bought F/A-18s instead. The US can afford the ridiculous fuel requirements of the M1's turbine engine, Poland, not so much, the Leopard's diesel is good enough. Smaller items like ammunition, radios and fuel type, that is far easier to make common, as they are less subject to national or regional requirements. And, indeed, commonality is not uncommon in those areas. So making an EU-standard vehicle or ship likely isn't going to work either.

So what is the problem that the OP's question is trying to solve?